This invention relates to improved processing for continuous or batch production of a metal or alloy from one or more compounds of that metal. Usually a halide compound dissolved in molten salt reacts with molten magnesium floating on molten salt. The invention was specifically designed for titanium production but use of the process is expected also for other metals, especially those for which Kroll is used, either as calcium reductions of oxides or magnesium reductions of halides.
Depending on conditions these products may be formed as metals or alloys, liquids or solids. Chemical and physical guidance of product formation may lead to structures including crystalline powder, powder agglomerates, and single crystal needles in various sizes. Such production has economic value in lowered costs relative to present production, in improved metal product quality, in supplying special needs, and in safer and environmentally improved operations, as compared with production by variations of the Kroll and Ames processes, e.g., respectively titanium or uranium.
An example of special needs is metallic needles for metal-organic composites for automobile panels. In using the present invention, the production of crystals of a metal involves the reduction of one or more compounds of that metal dissolved in a molten salt phase. With proper conditions for a given metal, it may be possible to grow needles or other useful shapes of that metal using the invention.
Prior Art:
Word Usage:
Commercial terms used regarding the Kroll process in various forms, e.g., in titanium production, are in some ways confusing. The industry's term "sponge" may be used more or less interchangeably with "powder" for describing the original Kroll process product, which may resemble a dirty clinker, and also for describing derivatives from the original product that form after crushing the process product and after cleaning it.
Usage in this disclosure may include identifying terms like "Kroll sponge" for the uncleaned product and "cleaned Kroll sponge" after substantial removal of the reaction by-product. Fine particles of metal product from this invention may be described by general terms: powder (with individual particles often made up of many small crystals); crystals (with various shapes depending on which faces of a crystal grew); and needles (often single crystals deposited electrolytically as the metal was forming during reduction of ions of the metal).
Older Related Art in Production of Titanium and Other Metals:
Kroll:
Conventional commercial production of titanium almost entirely utilizes pressure vessels for erratic, one-stage, Kroll batch reductions: For example, titanium tetrachloride (TiCl.sub.4) as gas, and excess molten magnesium metal react in a sealed reduction vessel at about 800-1000.degree. C. to form titanium "sponge." Reactions such as excess calcium with ZrO.sub.2 to form zirconium sponge and CaO have also been used widely in Kroll form. These reactions typically yield rather poor, expensive, often hazardous products that may, or may not, be suitable to clean to an adequate product. Many metals can be made in batch versions of this process, however.
In particular, the Kroll sponge is agglomerated metal particles that, when cooled, hold trapped by-products, such as magnesium chloride, excess magnesium, and impurities, e.g., magnesium oxide, TiCl.sub.2, and other metals. The acid and water washes originally used for Kroll cleaning are now inadequate. Much of the Kroll reduction by-product content can be removed by high-temperature vacuum distillation from the relatively nonvolatile titanium; later alloy melting can settle out some impurities and stir the melt.
Kroll and this Invention:
Metallic scrap or products of Kroll-type reductions might be recycled or purified by use in the invention of this application. Such treatments might increase the overall U.S. rate of quality product metal formation, as well as providing a way in which otherwise effectively unremovable impurities could actually be removed--both treatments have economic value.
This usage has not been taught previously, and it is an unobvious application of the present invention.
Ames:
The Ames process has been used for most production of uranium: In one-stage batch reductions, magnesium reacts with UF.sub.4 (not a gas) at about 1400.degree. C. to form molten uranium. It is recognized that Ames processing should be replaced.
Elliott, et al., Uranium; Replacing the Ames Process:
The first continuous molten salt-molten metal processing by metallothermic reduction for molten uranium or its alloys was invented by Elliott, U.S. Pat. No. 4,552,588, the present inventor, and coworkers at his laboratory, intermittently using Federal and private funds. This work was especially for depleted uranium tank armor but also for other needs. (Ames is batch only.)
Further development and demonstration at Elliott's laboratory and at Oak Ridge National Laboratory led to an improved form as taught in U.S. Pat. No. 5,421,855 for use with enrichment of natural uranium for commercial nuclear electric power
This older invention cannot handle volatile reactants like UF.sub.6 ; it is single stage, like Ames, and it requires low volatility reactants like UF.sub.4 and UCl.sub.4.
Earlier Alternatives to Kroll:
Hunter electrolysis was long used for titanium production, but in the U.S. it proved non-economic relative to Kroll and was shut down. Sodium reductions can provide excellent titanium but are too expensive for all but small markets and are mostly done outside of the U.S. Other approaches to titanium production include an early iodide decomposition process, newer approaches including Japanese electrolysis (which may become commercial), plus high temperature vapor reductions, and dehydriding.
TiCl.sub.2, an intermediate used in the present invention, is also an intermediate with Hunter and in sodium reductions; however, techniques to form and use the TiCl.sub.2 are not obviously related to the present invention.
Current Related Art:
Continuous Stirred Tank Reactor (CSTR) to Improve over Kroll:
White, et al., U.S. Pat. No. 5,259,862 invented a second continuous approach (CSTR) to continuous metallothermic production of metal (after U.S. Pat. No. 4,552,588). That system is now moving toward commercial usage.
U.S. Pat. No. 5,259,862 dissolves sodium or other reductant metal into molten salt, and mixes that salt with another solution that holds suspended titanium and has TiCl.sub.4 vapor bubbling up. It operates at approximately steady state with TiCl.sub.4, TiCl.sub.2, Ti, and dissolved sodium, all in the same stirred bath. Technically it is one-stage because it is one big bath, but it also provides regions where various steps go on.
U.S. Pat. No. 5,259,862 and This Invention:
U.S. Pat. No. 5,259,862 appears to this inventor to have an economic future in rough parallel, though apparently not as broad usage, as the future for this present invention. The two appear to be complementary in satisfying industrial needs.
Although dissolved magnesium is claimed for use in U.S. Pat. No. 5,259,862, sodium is presumably the obvious choice for reductant metal, with magnesium marginal at best there. In contrast, magnesium is generally the preferred reductant with the present invention; it operates with a magnesium as a second phase.
Magnesium Reduction of TiCl.sub.4, and Other Species to Replace Kroll:
Although uranium equipment claimed in U.S. Pat. No. 4,552,588 has been around for 12 years and was adapted (U.S. Pat. No. 5,421,855) for use with AVLIS uranium enrichment, it was not obvious to this inventor or to those versed in the art, that U.S. Pat. No. 4,552,588 had relevance for production of Ti from TiCl.sub.4.
Two-Stage Magnesium Reduction of TiCl.sub.4 to Replace Kroll Reductions:
To arrive at an alternative production approach that will correct Kroll's problems, it is first useful to analyze the Kroll reaction: EQU TiCl.sub.4(g) +2Mg.sub.(l) =Ti.sub.(s) +2MgCl.sub.2(l) (1)
Please note the following facts: (a) The MgCl.sub.2 by-product wets and coats the molten magnesium. (b) TiCl.sub.4 gas does not dissolve in molten MgCl.sub.2. (c) MgCl.sub.2, therefore, obstructs the main reduction reaction, Eq. 1.
To solve this incompatibility problem, this invention offers two reaction stages operating smoothly in a molten-salt medium: In one stage (named Stage 2) TiCl.sub.4 is formed in a solution of molten salt; unlike TiCl.sub.4, the TiCl.sub.2 dissolves readily in the molten salt. The chemistry is discussed later.
In another stage (named Stage 1) TiCl.sub.2 in molten salt reacts with magnesium floating on molten salt to form the products of Eq. 1.
The invention of Stage 2 is new and unobvious, and new equipment concepts had to be devised. Concepts similar to earlier uranium equipment were also adapted to Stage 1: however, the equipment for connecting Stage 1 and Stage 2 and for continuous cycling of the molten salt are all new. Also, the means of removing the product by a screw mechanism, and the by-product MgCl.sub.2 on a cool probe, are both new.
The present invention teaches two new aspects of continuous metal production: (i) It teaches two stage reductions in one continuously operating system for continuous formation of metal product forms, e.g., as molten or solid metals or alloys. (ii) It teaches regarding ways to control chemical and physical conditions that also can lead to guidance of formation of particular solid forms, e.g., crystal needles.
Thus reductions with only Stage 1 are adequate for the earlier usage (U.S. Pat. Nos. 4,552,588 and 5,421,855) because ionic UCl.sub.4 or UF.sub.4 would not boil away during the reduction. Stage 2, however, is required for chemical behavior like that of TiCl.sub.4 of this invention, and it leads to new claims that give an improved process over that taught and claimed by Hayden U.S. Pat. No. 5,421,855 and other Elliott patents.
This two stage improvement on earlier teaching appears to solve titanium problems that have been recognized for at least 30 years.
Also, a problem of continuous UF.sub.6 to U conversion may be soluble by similar treatments following this invention as some 600,000 metric tons of depleted UF.sub.6 by-product are brought from outdoor Federal storage and into appropriate control.
Additionally, the commercial value of producing needle-like crystals, possibly single crystals, of titanium or of other metals or alloys was not taught earlier: Elliott U.S. Pat. No. 4,552,588 notes that, when molten salt that includes dissolved uranium ions is in contact with molten magnesium at temperatures below the uranium melting point, there form "small crystals of solid uranium which sink and form small uranium droplets" in hotter regions of the uranium production system.
However, it was not taught that physical and chemical control of the reaction offered a potential method of producing useful needle-like crystals of pure uranium. Again, this is an unobvious method of forming metallic crystals, especially needle-like crystals.
Therefore, this disclosure now claims controlling physical and chemical conditions so as to guide the preparation of desired shapes, e.g., small crystals, single crystals, or both, of a desired metal by reduction of dissolved ions under particular chemical and physical conditions.